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Science

Variation and inheritance

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Genetic diagrams- Higher tier

Genetic diagrams are used to show the possible outcomes of a particular cross. A dominant allele is shown by a capital letter, and a recessive allele by a lower case letter.

Cystic fibrosis

Cystic fibrosis is an inherited disorder caused by a recessive allele. This genetic diagram shows the possible outcomes when both parents are heterozygous for the faulty allele. There is a one in four chance of the offspring being homozygous for the faulty allele, and so having cystic fibrosis.

f is the cystic fibrosis allele. Mother and father both have one F and one f allele. The four combinations of allele are: FF, Ff, Ff, ff.

Mother and father both have one F and one f allele

This genetic diagram shows the possible outcomes when only one parent carries the faulty allele. There is no chance of the offspring being homozygous for the faulty allele and therefore having cystic fibrosis.

f is the cystic fibrosis allele. Mother has two F alleles, father has one F and one f allele. The four combinations of allele are: FF, FF, Ff, Ff.

Mother has two F alleles, father has one F and one f allele.

Gender determination

Female gametes (eggs) contain X chromosomes, while male gametes (sperm) contain either X chromosomes or Y chromosomes. This genetic diagram shows that equal numbers of male and female offspring should be produced.

Gender determination

Females are XX and males are XY

Females are XX and males are XY.

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